The presence of power electronics devices on the power grid can cause power quality problems such as harmonic distortion and power system unbalance. Shunt active power filters can be employed to eliminate these power quality problems. Some control systems for the filters do not operate effectively under unbalanced power system conditions. Traditional methods of extracting the signals utilized in instantaneous reactive power theory (IRPT) have weak performance under unbalanced conditions. Therefore, a new extraction method is introduced with improved performance under unbalanced conditions. An improved control method, based on the proposed extraction method, is also presented. Simulation and experimental results validate the improved performance of this control method.